Security and scalability are considered as two major issues that are most likely to influence rapid deployment of blockchains in businesses. We believe that the ability to scale up a blockchain lies mainly in improving the underlying technology rather than deploying new hardware. Though recent research works have applied sharding techniques in enhancing scalability of blockchains, they do not cater for addressing the issue of both data security and scalability in blockchains. In this paper, we propose an approach that makes a trade-off between security and scalability when designing blockchain based systems. We propose an efficient replication model, which creates dynamic sharding wherein blocks are stored in a varying number of nodes. The proposed approach shows that the replication of blockchain over peer-to-peer network is minimized as the blockchain's length evolves according to a replication factor to preserve the security.